1. Field of the Invention
The present invention relates to optical information recording media of the heat mode type which each have a thin film of an organic dye and wherein recording and readout are conducted by causing a change of state with high density energy beams
2. Description of the Prior Art
Heretofore, information recording media wherein recording and readout of information are each carried out by irradiating a rotating disc-shaped information recording medium with a laser beam have been known. As recording layers in these information media, those wherein metals having low melting points, or metals having low melting points and dielectric substances are used have been proposed. However, these recording layers have disadvantages such as poor preservability, low resolution, low recording density and high manufacturing cost. Recently, it has been proposed and practiced that a thin film of an organic dye whose physical properties may be changed with light of relatively long wavelength is used in the recording layer. However, dyes which have absorption bands in long wavelength generally have problems, for example, a problem that they have only low stability against heat and light. Thus, it is the present state of the art that recording layers having recording characteristics which are stable over a long time period and are satisfactory have not yet been developed.
On the other hand, an optical information recording medium which contains an oxonol dye, particularly an oxonol dye having a dioxopyrazolopyridine ring has been disclosed in Japanese Patent Unexamined Published Application (hereinafter referred to as "J.P. KOKAI") No. 60-71295. However, dyes used in these information media are difficult to be synthesize, and have a great practical disadvantage that it is generally difficult to obtain them in a high purity and in a high yield. Further, only one compound among those disclosed in J.P. KOKAI No. 60-71295 is conjectured to be capable of having absorption maximum in the wavelength range of a semiconductor laser (no specific absorption maximum wavelength and example are disclosed therein about the compound). As is understood from the foregoing, it has been difficult to synthesize a dye having an absorption maximum wavelength in the wavelength range of a semiconductor laser, namely in the near infrared region, and it has been extremely difficult to synthesize and isolate such a dye in an amount necessary for evaluating its performance as an optical information recording medium.